Throughout the disclosure and claims, we will be using the term MPEG (Motion Pictures Expert Group). MPEG is a generic reference to a family of international standards, which define how to encode visual and audio information in a digital compressed format.
MPEG is utilized in a wide variety of applications, including: DVD (Digital Video Discs) and DVB (Digital Video Broadcasting).
The MPEG standards specify exactly the format in which the compressed data is to be transmitted. A key feature of MPEG is that it can compress a video signal into a fraction of its original size. MPEG achieves a high compression for video by storing only the changes from one video frame to another, instead of each entire frame.
There are two major MPEG standards: MPEG-1 and MPEG-2. The most common implementations of the MPEG-1 standard provide video quality slightly below the quality of conventional VCR (Video Cassette Recorder) videos. MPEG-2 provides higher resolution, with full CD quality audio. This is sufficient for the major TV standards, including NTSC (National Standards Television Committee) and HDTV (High Definition Television.
Of the series of MPEG standards that describe and define the syntax for video broadcasting, the standard of relevance to the present invention is ISO/IEC IS 13818-2, ITU-T Recommendation the MPEG-2 standard, titled “Generic Coding of Moving Pictures and Associated Audio Information: Video.”, which is incorporated herein by reference and is hereinafter referred to as “the MPEG-2 Standard”
Additional standards incorporated herein by reference are:
1) MPEG-2 Systems Group. Information Technology—Generic Coding of Moving Pictures and Associated Audio: Part 1—Systems, ISO/IEC 13180-1 International Standard 1995.
2) MPEG-2 Video Group. Information Technology—Generic Coding of Moving Pictures and Associated Audio: Part 2—Video ISO/IEC 13818-2 International Standard, 1995.
If an MPEG stream is to be viewed immediately as it is received, the communication channel must have enough bit rate capacity to provide the series of pictures at a real-time rate. Bit rate is the number of digital bits which a communication channel can transmit per second. Alternatively, pictures can be encoded to a size suitable for a channel of a given bit rate. MPEG encoding allows the size of each picture to be adjusted by varying quality; smaller pictures may be achieved at the expense of lower quality. The objective of an MPEG encoding is to maximize quality for the available bit rate.
Consider a system in which high-quality MPEG video is to be played directly from a storage medium; DVD is an example. The video quality is high, as instantaneous bit rate is of relatively low concern. Consider now that the stored high-quality video is to be communicated across a channel of constrained bit rate such as a telephone line. Some device must re-encode each picture (with potential reduction of quality) so the sequence of pictures may be transmitted in real time within the available bit rate.
A transcoder is a device which converts an encoded bit stream of one bit rate to a lower bit rate, and changes the content of the encode bit stream. This device is useful when down-stream channels operate at a lower rate than the up-stream channel.
An example would be the retrieval of data from a DVD drive at very high bit rate (and high quality) and transmission over phone line at a lower rate (and lower quality). The manner in which picture quality is traded-off for bits is a matter of transcoder design. Ideally, changes are made where they are least likely to be noticed by the viewer.
Traditional film mediums utilize the 35 mm format. This format provides a 4:3 aspect ratio. An aspect ratio is the ratio of width to height of an image on a television, motion picture, or computer screen. As movie making technology developed, a wide screen format became more popular. This format is often referred to as “Cinemascope”, “wide screen” or “letterbox”. This format has an aspect ratio of 16:9.
When digital video is stored on a high-speed storage device, such as a DVD, the bit rate of the stream is often less important than the quality and inclusion of special features. One such special feature is the recording of a movie in a 16:9 aspect ratio, which doesn't use up all of the screen area of a standard TV (4:3 aspect ratio). When viewed on a standard TV, a letterbox movie will have black bars at the top and bottom of the television screen, as a result of scaling down the image to fit the screen.
The MPEG-2 standard provides a means for embedding “pan-scan” information in a video bit stream. Pan-scan information defines which rectangular area of the stream is to be displayed for an aspect ratio different from the original recording.
U.S. Pat. No. 5,638,130, discloses the use of pan-scan conversion, but after the MPEG-2 stream has been decoded. This requires that the MPEG-2 stream be decoded prior to selecting the pan-scan conversion. This does not address the issue of reducing bit rate requirements between the source and the end consumer.
There is thus a need for a transcoder capable of reducing the bit rate of an MPEG stream between the source of the MPEG stream and the final consumer. The present invention addresses this need.